The present invention relates to plasma technology in class H05H of International Patent Classification.
FIG. 1 of the accompanying drawings illustrates a conventional high-frequency plasma torch T with a grounded carbon rod. The plasma torch T comprises a double-walled tubular torch body 100 composed of a larger-diameter outer tube 101 made of a heat-resistant material such for example of quartz and a smaller-diameter inner tube 102, a high-frequency induction coil C wound around the tubular torch body 100, and an ignition device L.
The outer tube 101 has a closed end and an opposite open end opening into a chamber ch for introducing a high-frequency plasma generated in the torch body 100 into the chamber ch to effect various actions therein. The inner tube 102 also has a closed end and an opposite open end, and extends through the closed end of the outer tube 101. The inner tube 102 includes a portion closer to the closed end, disposed outside of the outer tube 101, and a portion closer to the open end, disposed within the outer tube 101. The inner tube 102 is disposed concentrically with the outer tube 101 with the open end of the inner tube 102 being open in the outer tube 101. Conduit tubes open respectively into the outer and inner tubes 101, 102 in the vicinity of the closed ends thereof. The conduit tube connected to the inner tube 102 serves to supply a core gas G1 to be formed into a plasma into the inner tube 102, and the conduit tube connected to the outer tube 101 serves to supply a tube wall cooling gas G2 which is the same quality as that of the core gas G1, for example.
The high-frequency induction coil (hereinafter referred to as a "coil") C is wound around the torch body 100 adjacent to a position where the inner tube 102 opens in the outer tube 101 and where the torch body 100 is single-walled, and is connected to a high-frequency power supply E which produces an output of a predetermined frequency.
The ignition device L comprises an ignition carbon rod Lb of a small diameter, a sheath Ls by which the carbon rod Lb is supported, and a conductive wire Lc by which the carbon rod Lb is grounded. The carbon rod Lb is fitted in the sheath Ls such that a certain length of the carbon rod Lb is exposed out of the sheath Ls. The sheath Ls extends coaxially through the closed end of the inner tube 102 such that the exposed carbon rod Lb has a distal end positioned in the portion of the torch body 100 where the coil C is wound. The conductive wire Lc has one end connected to an end of the carbon rod Lb which is located outside of the torch body 100, with the other end of the wire Lc grounded.
The theory of ignition according to the above conventional ignition arrangement with the grounded carbon rod is as follows: When a high-frequency current starts being passed through the coil C while the core gas G1 is being introduced into the inner tube 102, an eddy current is induced in the carbon rod Lb due to a magnetic flux generated by the coil C thereby to heat the carbon rod Lb. The heated carbon rod Lb promotes ionization of the core gas G1. The core gas G1 is then ignited by a sparking voltage--a silent discharge starting voltage due to an electric field generated radially of the coil between the ground ignition rod Lb of a lower potential and the coil C of higher potential.
In the conventional ignition arrangement with the grounded carbon rod, it is required to ground the carbon rod Lb.
With the prior ignition arrangement with the grounded carbon rod, the carbon rod Lb is fitted in the sheath Ls and has its distal end placed in the high-frequency energy region after ignition. Therefore, the distal end of the carbon rod Lb is always held in contact with the tip of a high-temperature plasma fire indicated by P in FIG. 1. Since carbon is melted and mixed into the plasma, it is difficult to obtain a highly pure plasma fire P consisting of the component of the core gas G1. Furthermore, a few seconds has been required until the ignition occurs.
Examples in which a highly pure plasma fire P is required are as follows: Where the core gas G1 is of N.sub.2 and the plasma fire P acts on a bulk of titanium (Ti) to produce fine particles of titanium nitride, 10 to 30% of the fine particles produced by the plasma torch T of a conventional ignition apparatus L is no made of titanium nitride but titanium carbide. Therefore, no desired purity of products can be ensured. Where a rod of Ti is used in place of the carbon rod, it is held in contact with the high-temperature plasma fire having at least a temperature of 10,000 K. as long as the prior ignition arrangement with the grounded rod, with the result that the rod material will be melted.